a) Field of the Invention
The present invention is a link mechanism to determine a position and direction in three dimensional space. Such a link mechanism is applicable to determine the position and direction of a surgical assist apparatus, an end effector of a robot (manipulator) or the like.
b) Description of the Related Art
In MR/T (magnetic resonance and therapy), fusion of diagnosis and treatment, it is necessary that the presence and function of a medical equipment do not generate artifacts (such as noises and ghost) on the image. The mutual influence between a medical equipment and MRI is called MR compatibility.
Surgical assist robots are expected to be used in MR/T. However, since robot mechanism generally uses many metal components such as steel components, electromagnetic motors, electric sensors and the like, it has been difficult to realize MR compatibility. In particular, the end effector, which is used near to a patient, is required to have a very small magnetic susceptibility and low electrical noise radiation. It is therefore difficult to use a complicated mechanism, active mechanical elements such as motors, and various types of sensors.
Under such restrictions, surgical assist robot for MR/T must realize a mechanism to establish, for a robotic arm segment, at least the position (x y z orthogonal coordinates) and angles θ and φ (measured from the center of a coordinate axes system as, respectively, azimuth and elevation) to define the direction in three-dimensional space. It is also desired that the end effector is cleanable and sterilizable. The end effector is also required to be simple and compact so as not to interrupt the view field and the work area of the surgeon. In addition, the output power of an actuator should be as small as possible to maintain safety.
There are many conventional mechanisms to define the position (in terms of the coordinate axes x y z) and angles θ and φ. A typical example of such mechanisms is an arm type robot. The arm type robot uses a number of arm links interconnected by joints.
The arm type robot usually has actuators at the joints, otherwise it employs wire mechanism, shaft, and so on to transmit the driving power. The actuator makes it difficult to maintain MR compatibility, and inhibits cleaning and sterilization unless a water-proof process is performed strictly. Since the transmission mechanism is composed from many parts and is mechanically complicated, it requires a number of power transmission components so that it is mechanically complicated and a power loss cannot be neglected.
A parallel link mechanism, typically a hexa pod type robot, can be designed to place actuators remotely from the end effector. However, since links are concentrated near to, or at the end effector, the mechanical structure is complicated.
By designing the end effector to be lengthy, it may be MR compatible, cleanable and sterilizable.
In this case, however, its precision is degraded and stronger actuators are required. Such an end effector can be complicated and bulky, and consequently interrupt the view field of the surgeons.
Under such circumstances, a link mechanism to determine the position and angles has long been desired, which mechanism has a high precision of position and angle determination, does not require strong actuators, and is mechanically simple for enabling MR compatibility, cleaning and sterilization.